A protective sheet or covering material is usually affixed to pressure sensitive adhesive coated materials to protect the adhesive coating. The protective sheet is rendered easily removable from the adhesive layer by providing the sheet with a coating of a material having good release properties. Ideally, one should be able to peel away the protective sheet without taking the adhesive coating with it and without destroying, even after long contact, the aggressiveness of the adhesive in its ultimate application. For example, in the manufacture of die cut printed labels, bumper stickers and double faced tapes having a layer of a pressure sensitive adhesive as a means for securing such items, a paper or plastic sheet having a release coating on one side has been employed to protect the adhesive layer prior to use. This is particularly true where the pressure sensitive adhesive coated substrates must be shipped, handled and stored on top of one another. Similarly, in the manufacture of pressure sensitive adhesive tapes the side of the tape opposite that to which the pressure sensitive adhesive is applied contains a coating of a release material to permit facile separation of the adhesive and the backing when the tape is unwound.
The ability to release the protective sheet from an affixed adhesive layer in a controlled manner is not the only requirement for a commercially acceptable release agent. If the release materials travel into the adhesive or remain on the adhesive surface, the tackiness of the adhesive is diminished. Since most adhesive coated substrate/release paper composites are shipped and stored for relatively long periods of time, frequently under high temperature and high humidity conditions, there is ample opportunity for deleterious migration to occur unless the release agent is securely "locked" at the surface of the release paper.
Currently used release materials include silicones, fluorocarbons, fatty acid/metal complexes and hydrocarbon waxes. The silicone and fluorocarbons suffer from their extreme high costs, difficulty in handling due to organic solvent or chemical instability and the tendency to give excessive release leading to mechanically unstable composites. The fatty acid/metal complexes have many of the same problems in addition to an inherently intense color which is frequently undesirable for aesthetic reasons. To overcome the cost factors these materials are usually applied in a two step process over an impervious barrier coat of polyvinyl alcohol or other material which prevents absorption into the paper and fills tiny surface voids. Hydrocarbon waxes, while being low in cost, tend to undergo cohesive failure leaving wax affixed to the adhesive with resultant losses in final adhesive performance. Thus the waxes are usually found only in low cost, nondemanding applications.
U.S. Pat. No. 2,491,642 discloses the use of polyethylene oxide as a stripping agent in polyvinyl alcohol compositions in the formation of polyvinyl alcohol films by continuous casting on a metal casting surface.
U.S. Pat. No. 2,985,544 discloses aqueous emulsions that can be used to render paper nonadherent to various organic solids. The emulsion compositions comprise (1) a linear polydimethylsiloxane having terminal silicone bonded hydroxyl groups, (2) a methylpolysiloxane resin, (3) polyvinyl alcohol as emulsifying agent and (4) dibutyl tin dilaurate. The dibutyl tin dilaurate gave minimal migration of the methylpolysiloxanes to the surface of the paper.
U.S. Pat. No. 3,257,348 discloses compositions for making chlorine resistant polyvinyl alcohol films comprising an incompletely hydrolyzed polyvinyl alcohol, a polyoxyethylene aryl ether and, optionally, an alkali metal salt of mixed phosphate esters of an ethoxylated higher alkyl phenol as a plasticizing agent.
U.S. Pat. No. 3,690,924 discloses a release coating comprising polyvinyl alcohol, a fluorochemical containing a nonpolar "head" portion of a polyfluorocarbon and a polar "tail" portion which is a carboxylic acid group of the Werner complex type with chromium, and an insolubilizer for polyvinyl alcohol. Insolubilizer materials can be metal salts, for example, chromic nitrate and sodium, potassium, ammonium and cupric dichromate; cuprammonium hydroxide and titanium lactate.
U.S. Pat. No. 3,933,702 discloses a release coating composition comprising a hydrocolloid stabilized-aqueous emulsion of a crosslinkable vinyl-type polymer and a release promoting compound such as organosilicone compounds. Polyvinyl alcohol may be employed as the sole emulsifying agent.
U.S. Pat. No. 4,049,860 discloses a paper support carrying an anti-adhesive layer which comprises polyvinyl alcohol, a polysiloxane and a polyalkylene glycol wax.
U.S. Pat. No. 4,142,014 discloses thermally resistant release coating compositions comprising solutions or emulsions of metal salts of hydrolyzed long chain alkyl vinyl ethermaleic anhydride copolymer. Metal oxides or hydroxides of metals such as sodium, potassium, lithium, copper, zinc, chromium, iron, lead and antimony may be used to form the salts.
U.S. Pat. No. 4,146,511 discloses a resin composition for peel-off coatings which contain a film forming polymeric resin such as polyvinyl alcohol, and organosiloxane having a polyoxyalkylene group in the side chain and a solvent.
U.S. Pat. No. 4,241,198 discloses a release coating agent containing a copolymer of (1) a monomer mixture comprising stearyl acrylate and/or stearyl methacrylate and (2) an ethylenically unsaturated compound having functional groups reactive with metal alcoholate, and a metal alcoholate for crosslinking the functional groups.
Nevertheless, there remains a need for an effective release coating which is relatively low in cost and can be applied from a water medium.
There is yet a need for an effective polyvinyl alcohol based release coating that is applied in a one step coating operation and does not adversely effect the readhesive qualities of a pressure sensitive adhesive coated substrate.
Further, there is a need for a polyvinyl alcohol based release coating that permits the release agents to migrate to the surface of the release coating without substantially travelling into the adhesive layer or remaining on the adhesive surface upon removal of the protective sheet containing the release coating.